There are many well known systems for recording and reproducing video signals. For example, an analog video cassette recorder (VCR) may be used to record analog video signals on a magnetic tape and to play back analog video signals that have been recorded on a magnetic tape. In order to prevent systems such as the VCR from being used to create unauthorized copies of copyrighted works, various copy guard techniques have been proposed. Two current copy guard techniques are the Automatic Gain Control (AGC) pulse technique and the color stripe technique.
In AGC pulse systems, pulses with an amplitude greater than the normal AGC reference level are placed in the AGC reference level detecting interval of a video signal. For example, as shown in FIGS. 1A and 1B, pseudo sync pulses are placed in the vertical blanking interval of a video signal (FIG. 1B is an enlarged view of a portion of the signal shown in FIG. 1A). In the depicted example, five pseudo sync pulses are inserted into the video signal at a point in time immediately following a horizontal synchronizing pulse and a color burst pulse. The amplitude of the inserted pulses is equal to the normal AGC reference level plus "p", where p is variable.
The effect of the inserted pulses is to interfere with a VCR's calculation of the AGC reference level, and thereby prevent satisfactory recording and playback of the video signal. As shown in FIG. 1C, many VCRs set the AGC reference level according to the amplitude of the horizontal sync pulse. However, when pseudo sync pulses are inserted, a VCR is "fooled" into using the inserted pulses to set the AGC reference level. Accordingly, as shown in FIG. 2A, the AGC reference level is erroneously determined to be the normal AGC reference level plus "p". The erroneously high AGC reference level causes the VCR's control circuits to greatly reduce the video gain. Consequently, as shown in FIG. 2B, the amplitude of the reproduced horizontal sync pulses is reduced to a level that is undetectable, and the video signal cannot be properly reproduced. TV monitors, however, use an AGC system that differs from that of analog VCRs and can adequately display reproduced images notwithstanding the pseudo sync pulses.
A drawback of the AGC pulse technique is that it does not always provide adequate copy guard protection. VCRs having long AGC time constants, such as .beta. system VCRs, 8 mm VCRs, and some VHS system VCRs, are not particularly sensitive to pseudo sync pulses. To account for this, the color stripe technique has been proposed.
In a color stripe system, the phase of the color burst signal in the reproduced video signal is inverted every N-out-of-M lines. For example, the phase of the color burst signal is inverted every 4 out of 21 lines. When a video signal processed by the color stripe system is recorded on an analog VCR, the Automatic Phase Control (APC) circuit of the VCR erroneously recognizes the phase-inverted color burst signals as original color burst signals. This results in an inversion of colors in the video lines having the inverted color burst phases. Consequently, when the signal recorded by the VCR is reproduced, color-inverted lines appear in the displayed image as shown in FIG. 3. TV receivers, however, typically use long time constant APC circuits, and thus the picture they generate is unaffected when the phase of the color burst signal is inverted in only four lines.
A drawback of the color stripe technique is that some TV receivers use small time constant APC circuits, and therefore when these receivers display signals processed by a color stripe system, the displayed image includes color-inverted lines.